A typical example of a plated steel sheet having favorable corrosion resistance is a galvannealed steel sheet. The galvannealed steel sheet is generally manufactured by degreasing a steel sheet, preheating the steel sheet in a non-oxidizing furnace or direct fired furnace, carrying out reduction annealing in a reduction furnace for cleaning the surface and securing the material quality, immersing the steel sheet in a galvanizing bath, controlling the adhered amount of molten zinc, and carrying out alloying. Since the steel sheet has excellent corrosion resistance, plate adhesiveness, and the like, the steel sheet is widely used mainly for automobiles, construction materials, and the like.
Particularly, in recent years, in order to both secure a function for protecting passengers in case of collision and reduce the weight for improving the gas mileage in the automobile field, there has been a demand for an increase in the strength of a plated steel sheet. However, generally, an increase in the strength results in degradation of the formability, and thus there has been a demand to establish a method for increasing the strength while the formability is maintained.
Examples of the method for increasing the strength while the formability is maintained include methods as described in Patent Documents 1 and 2. These methods are for increasing the strength and obtaining favorable formability at the same time by dispersing residual austenite in steel and using the fact that the residual austenite causes a stress induction and deformation induction during a process. In the steel sheet as described in Patent Documents 1 and 2, C, Si, and Mn are used as basic alloy elements, annealing is carried out in a two-phase region of ferrite (α)+austenite (γ), and then a thermal treatment is carried out in a temperature region of approximately 300° C. to 450° C., thereby using a bainite transformation and obtaining residual austenite even at room temperature. However, since carbides, such as cementite, tend to be precipitated during the thermal treatment of 300° C. to 450° C., and austenite is decomposed, it is necessary to add Si or Al.
However, since Si and Al are more liable to be oxidized than Fe, it is likely that oxides containing Si or Al are formed on the surface in the above steel sheet. These oxides have poor wetting properties with molten Zn, and thus, in steel sheets containing Si or Al, there is a problem in that non-plated portions are liable to be formed. In addition, the above oxides delay the alloying reaction between Zn and Fe. Therefore, in steel sheets containing Si or Al, long alloying treatment with a high-temperature is required compared with mild steel sheets, degradation of the productivity is caused, austenite is decomposed into a bainite structure including pearlite and carbides by long alloying treatment with the high-temperature, and an excellent formability cannot be obtained.
Patent Document 3 describes a method for solving the above problems. This method is for improving the wetting properties of steel sheets and molten Zn and accelerating the alloying reaction by adding an appropriate concentration of Al to molten Zn.
This method makes it possible to suppress propagation of fatigue cracking that propagates through soft ferrite by structural strengthening that strengthens soft ferrite using a hard structure, such as hard martensite or residual austenite, and thus this method contributes to improving the fatigue durability up to a certain fraction of hard phases. However, since fatigue cracking propagates through soft structures, there is a limit to increasing the fatigue limit simply with an increase in the fraction of hard structures. As a result, when the fraction of hard structures reaches a certain extent or more, the strength of the steel sheet is increased, but the fatigue limit is not increased. Therefore, it was difficult to achieve both an increase in the strength and fatigue durability to a high level (for example, refer to Non Patent Document 1).
Meanwhile, since steel sheets used for automobiles or construction materials have a thin sheet thickness, there are cases in which, when fatigue cracking is formed, the sheet thickness is immediately penetrated and fractured. Due to this fact, suppression of the formation of fatigue cracking is particularly important.
Ordinary techniques for improving the fatigue durability include a method of using precipitation strengthening (for example, refer to Patent Document 4). However, in order to use precipitation strengthening, it is necessary to heat the steel sheet to a high temperature sufficient to melt precipitates (for example, carbonitrides of Nb or Ti) and then cool the steel sheet, and therefore this technique can be applied to hot-rolled steel sheets, but it is difficult to apply the technique to cold-rolled steel sheets.
In addition, Patent Document 5 describes a technique that isolates and disperses a soft phase (ferrite) in a hard second phase, and controls the thickness of the hard phase to be greater than the value specified by the grain diameter of the soft phase, thereby improving the fatigue durability. However, this technique is for suppressing the propagation of cracking formed on the surface of the steel sheet, and is not for suppressing the formation of cracking on the surface, and therefore it is difficult to sufficiently improve the fatigue durability of the steel sheet using this technique.
In addition, Patent Document 6 describes a technique that controls the depths of grain boundary oxides in the interface between a plated layer and a steel sheet to 0.5 μM or less, thereby improving the fatigue durability. The reason why the fatigue durability is improved is considered to be because the decrease in the depths of the grain boundary oxides suppresses stress concentration in the interface between the plated layer and the steel sheet. However, even with this technique, it was difficult to sufficiently suppress the formation of cracking on the surface.
In addition, addition of Si to steel is carried out as an inexpensive method of strengthening a high-strength steel sheet. However, when the amount of Si in the steel exceeds 0.3% by mass %, there was a problem in that the wetting properties are significantly degraded in the Sendzimir method in which a plating bath containing ordinary Al is used, parts of the surface are not plated, and therefore the appearance quality is deteriorated. It is reported that the above phenomenon is because Si oxides are concentrated on the surface of the steel sheet during the reduction annealing, and the wetting properties of the Si oxides with respect to molten Zn are deteriorated.
As measures for solving the above problem, Patent Document 7 describes a method in which heating is carried out in an atmosphere having an air ratio of 0.9 to 1.2 so as to generate Fe oxides, the thicknesses of the oxides are controlled to 500 Å or less in a reduction zone in an atmosphere including H2, and then plating is carried out in a bath to which Mn and Al are added. However, in actual production lines, various kinds of steel sheets including a variety of added elements are threaded, which makes it very difficult to control the thicknesses of the oxides. In addition, Patent Documents 8, 9, and the like describe methods in which specific plating is carried out so as to improve the plating properties as other measures for suppression, but these methods require installation of a new plating facility to the galvanizing line ahead of the annealing furnace, or require a plating treatment that is carried out in advance in an electrical plating line, which significantly increases costs.
Steel sheets having a tensile strength of, ordinarily, 780 MPa or more and, recently, 980 MPa or more are used as high-strength steel sheets for the reinforcing members of automobiles. The high-strength steel sheet is formed generally by a process that is mainly intended for bending. It is known that a high-strength steel sheet having a high C concentration increases the hardness of the steel sheet itself, and the average hardness of the surface layer of the steel sheet, which is measured by the nano-indentation method, exceeds 3.5 GPa.
Here, the nano-indentation method refers to one of methods for evaluating the mechanical properties of thin films. In this method, a small needle is pressed to the measurement target of a thin film under a certain load, the ingression depth of the needle is measured with nanometer (nm) accuracy, and the property values, such as hardness or elasticity, of the thin film are computed.
A problem in a case in which a high-strength steel sheet having a high C concentration is used is hydrogen embrittlement. The hydrogen embrittlement is breaking that is caused by atomic hydrogen which has intruded in the grain boundary and the like under a tensile stress, such as a residual stress. The hydrogen embrittlement can be suppressed by a method in which the concentration of hydrogen in steel is decreased by carrying out a dehydrogenating treatment after the steel sheet is processed into a member, but the number of the manufacturing steps is increased, and the costs are increased.
In addition, when the surface layer of a high-strength steel sheet is hard, cracking is liable to occur in the surface layer of the steel sheet during the bending process, cracking develops during use, and the steel sheet is broken in the sheet thickness direction. This degradation of bending properties causes a significant problem. Patent Document 4 describes a method in which an isothermal treatment is carried out during the annealing step for improving the bending properties, but 3 minutes or more of the isothermal treatment is essential, and this method carried out using a continuous plating facility causes a significant degradation of productivity.
In Patent Documents 11 and 12, the structure or C concentration in the ferrite portion is controlled in order to improve the plating properties. These documents focus to the surface properties of the ferrite while plating is carried out. However, these documents do not sufficiently disclose the analysis, which is carried out after the plating, of the properties of the ferrite portion that directly adjoins the interface with the plate. In the method of Patent Document 12, it is difficult to measure the concentration of C immediately below the interface, particularly, at a depth of 1 μm or less.